Loud-speaker assembly



May 29, 1951 4c. E. HoEKs'rRA ET AL. 2,554,859

LOUD-SPEAKER ASSEMBLY Filed Oct. l, 1949 Patented May l29', 1951 assista LOUD-SPEAKER ASSEMBLY Cyrus E; Hoekstra and John C. Radcliffe, Fort Wayne, Ind., assignors to The Magnavox Company, Fort Wayne, Ind., a corporation of Dela- Ware Application October 1, 1949, Serial No. 119,118

9 claims. (c1. 1794-1155) The present invention relates to electromechanical transducers, and its teachings are particularly useful in the production of permanent magnet radio speakers of the moving voice coil type.

It is the general aim of the invention to provide an improved structure for the magnetic circuit of a loud speaker, whereby the eciency of the magnetic circuit employed is increased substantially above the maximum efficiency possible with those previously used. In furtherance of the general aim, it is the primary object of the invention to provide a magnetic circuit for a movture in a soft iron pole plate and completing the circuit by a return yoke from the remote end of the magnet to the plate. It has been learned, however, that even with the most powerful permanent magnet alloys presently available, the

the same diameter as the voice coil fails to provide the desired iiux density at the air gap, even vwhen the end of the permanent magnet is extended directly into the voice coil aperture, and

the pole plate and the flux return path of the circuit are formed of the best grade of soft magnetic materials. It has, therefore, become com- Ymon practice to employ permanent magnets of considerably larger diameter than the diameter of the voice coil, so that the increased cross sec-r tional area of the magnet provides more lines of -force across the air gap. Tapered magnets have been suggested, but it has been a more common expedient to employ a permanent magnet in the form of an enlarged slug housed within the,

utilization of a single cylindrical bar magnet cfg,

speaker yoke, and provided with a tapered or flanged pole piece extending into the ofce coil. Fairly satisfactory results may be obtained by such structures, but they are inefcient in utilization of the flux of the permanent magnet, andconsequently require magnets of considerably larger diameter than the voice coil in order to bring about the desired degree of flux density in the gap in which the voice coil moves.

The present teachings depart from prior practice by providing, in combination with a pole plate and return circuit of magnetically permeable material, an alloy permanent magnet and a magnetically soft pole piece of unique dimensional relations found to give a marked increase in efliciency of the circuit and to provide increased flux density in the air gap with any given weight of permanent magnet material. It has been found by actual test that whereas in a conventional speaker it is possible to get only about one-third of the magnetic ux of the magnet to thread the air gap, with the present invention about two-thirds of the flux will thread the gap. This is due to the fact that the permanent magnet employed, while of small diameter and extending into the voice coil, is not itself used as the pole piece. Instead, it is provided with a pole piece of magnetically permeable soft iron, mild steel or similar magnetic material, with the pole piece thinner than the pole plate at the air gap and located between its opposite faces.

This arrangement takes full advantage of the inherent magnetic characteristics of alloys such as Alnico V and "Alnico VI, in which the grain is oriented in its heat treatment by holding it in a magnetic field during one of its cooling cycles so that its magnetic properties are greater along its axis than in any other direction. That is, its properties are extremely directional in nature. Thus, if the magnet is used as the pole piece, the lines of force extending across the air gap are perpendicular to the magnetic axis of the magnet, and the rleld is comparatively weak. In the present teachings, this is avoided by the use of a pole piece of soft iron, which not only reduces the weight of expensive magnetic alloy required, but results in an important gain in flux density vin the gap, particularly when the pole piece is dimensioned and positioned in the specific manner here disclosed. In short, the present applicants have discovered that by providing a magnetically permeable pole plate and concentric pole piece which is substantially thinner than the pole plate and wholly within the aperture, it is possible to obtain such an efficient concentration of lines of force across the gap that it is entirely feasible to utilize a simple, cylindrical permanent magnet of diameter smaller than the voice coil.

A preferred embodiment of the invention is illustrated in the drawing of this specification wherein a radio speaker comprising a present commercial application of these teachings is illustrated. The single feature of the drawing is a fragmental cross sectional view through the speaker assembly.

The magnetic circuit of the speaker in which the novelty of the present invention resides includes a cylindrical magnet l0, a pole piece il, a pole plate I2 and a yoke or pot I3. The magnet ld is an aluminum-nickel-cobalt-iron alloy,

.Which is precipitation hardened and manufactured by a process wherein its grain is oriented during its heat treatment by holding it in an intense magnetic eld as it cools. This is an alloy commonly employed in speaker magnets and widely sold in the trade under the name of Alnico V. The pole piece II, pole plate I2 and yoke I3 are of what is ordinarily termed soft iron in the art, the term as used here including all equivalent magnetically soft material. Excellent results are obtained by the use .of S. A. E. 1010 (or lower carbon) hot rolled steel, though steel or iron of other carbon content may be employed, as may other magnetic materials such as sintered powdered iron shapes.

The magnetic axis of the magnet I extends along the line A-B, which is concentric with the cylindrical walls of the magnet and with the other operating parts of the speaker. The rear face surface I4 of the magnet is ground smooth and is seated flat against a fiat face I5 on the inside of the back wall I0 of the yoke I3. The magnet is held in position by any convenient means, as by solder Il. The forward face I8 of the magnet is in nat surface contact with the back face I9 of the pole piece II, and these two parts are preferably secured together by solder introduced through a central opening 2I. The pole plate I2 is of considerably greater thickness than the pole piece iI, so that its inside face 22 lies in a plane oifset from and spaced rearwardly of the back face I9 of the pole piece. while its outside face 23 lies in a plane offset from and spaced forwardly of the front face 4 spaced forwardly from the pole plate face 22 about .060.

In the process of manufacture the plate I2 and yoke I3 may be joined in any conventional Way or, if desired, these parts may be formed as a single integral piece. As shown, the pole piece and magnet are soldered in position and are preferably magnetized during the soldering operation, so that the abutting face surfaces at each end of the magnet are held in intimate surface contact to give minimum magnetic reluctance at these joints. The soldering operation may conveniently be accomplished -by inserting a non-magnetic ring gauge in the air gap and fusing the solder I1 and 20 simultaneously by heat supplied by a hot plate.

Before assembly, a dust guard 26 is inserted as shown. The speaker is completed by attaching a basket 2T in the usual manner and mounting the spider 28 and voice coil form 29 on which the voice coil windings 3l are carried. The speaker cone 32 is mounted on the outer rim 33 of the basket with its apex joined to the voice coil form and spider in the usual manner. A dust cap 34 may also be secured in position as shown to prevent entry of dirt and magnetic particles into the Voice coil air gap.

From the foregoing it will be seen that the teachings of this disclosure provide two magnetic poles of novel form, which coact to provide maximum flux density in an annular air gap. The outer magnetic pole consists of an internal cylindrical surface 35 extending between parallel annular edges at the rear and front faces 22 and 23 of the pate l2. The inside pole. consists of the thin disc II, which has a cylindrical pole surface 3S extending between annular edges at the front and back face surfaces 24 and I9. This structure departs from prior practice in several well dened particulars, primarily in that (l) the pole piece is wholly within the voice coil aperture; (2) the pole piece is spaced rearwardly of the front face and forwardly of the rear face of the plate; and (3) the magnet l0 extends substantially to (or somewhat beyond) the rear face 22 of the plate. It has been learned that by these expedients the efciency of the mag;` netic circuit is improved to a marked extent. Instead of having the magnetic flux in the air gap approximating only one-third of the flux threading the magnet (as is usual with conventional speakers), approximately tWo-thirds of the total ux threads in the air gap. This improvement in performance arises from the specic positional relationships of the parts shown, in which the magnetically soft pole piece I I is formed of metal substantially thinner than the pole plate I2, with its surfaces both at the front and rear being offset inwardly from the faces of the pole plate, rather than ush or protruding as in prior teachings. The permanent magnet I0 may extend slightly into the aperture of the pole plate, but does not itself function as the `pole piece. Rather, it energizes the permeable magnetic material of the Dole piece Il, so that the highly directional magnetic characteristic of the permanent magnet alloy is utilized to its fullest extent, the lines of force passing from the magnet to the pole piece in the direction of the magnetic axis rather than at right angles thereto. The magnet diameter is smaller than the voice coil, and extends into the pole plate aperture sufficiently to support the pole piece in correct position therein.

Having thus described the invention, what we claim as new and desire to secure by United States Letters Patent is: g

1. A magnetic circuit for a transducer com prising, in combination, a pole plate of magnetically soft iron having relatively flat front and back face surfaces and a voice coil aperture with an internal cylindrical pole surface therein terminating in parallel annular front and back pole plate edges at said face surfaces; a magnetically soft iron pole piece comprising a circular disc having flat parallel front and back face surfaces and an external cylindrical pole surface terminating in parallel annular edges at said face surfaces; said pole piece being concentric with the voice coil aperture and of thickness less than the pole plate; the pole piece being positioned wholly within the voice coil aperture; with the front face surface of the pole piece spaced rearwardly from the front face of the pole plate and the rear face surface of the pole vpiece spaced forwardly from the rear face of said pole plate; a return yoke of magnetically soft iron including a back plate parallel with and spaced from said pole plate, and a magnetic generator comprising a cylindrical permanent magnet of diameter less than the diameter of the voice coil aperture; said magnet extending from the back plate of the return yoke into the aperture, and abutting the back face surface of the pole piece.

2. A magnetic circuit for a transducer comprising, in combination, a pole plate of magnetically soft iron having relatively flat front and back face surfaces and a Voice coil aperture with an internal cylindrical pole surface therein terminating in parallel annular front and back pole plate edges at said face surfaces; a magnetically soft iron pole piece comprising a circular disc having at parallel front and back face surfaces and an external cylindrical pole surface terminating in parallel annular edges at said face surface; said pole piece being concentric with the voice coil aperture and wholly within the voice coil aperture; a return yoke of magnetically soft iron including a back plate parallel with and spaced from said pole plate, and a magnetic generator comprising a cylindrical permanent magnet of diameter less than the diameter of the voice coil aperture; said magnet extending from the back plate of the return yoke and abutting the back face surface of the pole piece.

3. A magnetic circuit for a transducer cornprising, in combination, a pole plate of magnetically soft iron having relatively flat front and back face surfaces and a voice coil aperture with an internal cylindrical pole surface therein terminating in parallel annular front and back pole plate edges at said face surfaces; a magnetically soft iron pole piece comprising a circular disc having flat parallel front and back face surfaces and an external cylindrical pole surface terminating in parallel annular edges at said face surfaces; said pole piece being concentric with the voice coil aperture and of thickness less than the pole plate; the pole piece being positioned wholly within the voice coil aperture; with the front face surface of the pole piece spaced rearwardly from the front face of the pole plate and the rear face surface of the pole piece spaced forwardly from the rear face of said pole plate; a return yoke of magnetically soft iron including a back plate parallel with and spaced from said pole plate, and a magnetic generator comprising a permanent magnet of aluminum-nickelcobalt-iron alloy; said magnet having parallel nat rear and forward pole faces, with the rear pole face abutting the back plate of the return yoke and the forward pole face extending beyond the rear face of the pole plate, into the aperture, and abutting the back facesurface of the pole piece.

4. A magnetic circuit for a transducer comprising, in combination, a pole plate of magnetically soft iron having relatively flat front and back face surfaces and a voice coil aperture with an internal cylindrical pole surface therein terminating in parallel annular front and back pole plate edges at said face surfaces; a magnetically soft iron pole piece comprising a circular disc having flat parallel front and back face surfaces and an external cylindrical pole surface terminating in parallel annular edges at said face surfaces; said pole piece being concentric with the voice coil aperture and positioned wholly within the voice coil aperture; a return yoke of magnetically soft iron including a back plate parallel with and spaced from said pole plate, and a magnetic generator comprising a permanent magnet of aluminum-nickel-cobalt-iron alloy; said magnet having parallel flat rear and forward pole faces, with the rear pole face abutting the back plate of the return yoke and the' forward pole face abutting the back face surface of the pole piece.

5. A magnetic circuit for a transducer comprising, in combination, a pole plate of magnetically soft iron having a voice coil aperture with an internal cylindrical pole surface therein terminating in parallel annular'front and back pole plate edges; a magnetically soft iron pole piece of circular shape having an external cylindrical pole surface terminating in parallel annular front and back edges; said pole piece being concentric with the voice coil aperture and of thickness less than the pole plate; the pole piece being positioned wholly within the voice coil aperture; with the front edge of the pole piece spaced rearwardly from the front edge of the pole plate and the rear edge of the pole piece spaced forwardly from the rear edge of said pole plate; a return yoke of magnetically soft iron, and a magnetic generator comprising a cylindrical permanent aluminum-nickel-cobalt-iron alloy magnet of diameter less than the diameter of the voice coil aperture; said magnet extending from the return yoke into the aperture, and to the pole piece.

6. A magnetic circuit for a transducer comprising, in combination., a pole plate of magnetically soft iron having relatively at front and back face surfaces and a voice coil aperture with an internal cylindrical pole surface therein terminating in parallel annular front and back pole plate edges at said face surfaces; a magnetically soft iron pole piece comprising a circular disc having flat parallel front and back face surfaces and an external cylindrical pole surface terminating in parallel annular edges at said face surfaces; said pole piece being concentric with the voice coil aperture and positioned within the voice coil aperture with the rear face surface of the pole piece spaced forwardly from the rear face of said pole plate; a return yoke of magnetically soft iron including a back plate parallel with and spaced from said pole plate, and a magnetic generator comprising a cylindrical permanent' aluminum-nickel-cobalt-iron alloy magnet of diameter less than the diameter of the voice coil aperture; said magnet having parallel flat rear and forward pole faces, with the rear pole face abutting the back plate of the return yoke and the forward pole face extending beyond the rear face of the pole plate, into the aperture, and abutting the back face surface of the pole piece.

7. In a magnetic circuit for a transducer, the sub-combination of a pole plate of magnetically soft iron having a voice coil aperture with an internal cylindrical pole surface therein terminating in parallel annular front and back pole plate edges; with a magnetically soft iron pole piece of circular shape having an external cylindrical pole surface terminating in parallel annular front and back edges; said pole piece being concentric with the voice coil aperture and of thickness less than the pole plate; and positioned wholly within the aperture with the front edge of the pole piece spaced rearwardly from the front edge of the pole plate and the rear edge of the pole piece spaced forwardly from the rear face of said pole plate.

8, In a magnetic circuit for a transducer, the sub-combination of a pole plate of magnetically soft iron having relatively flat front and back face surfaces and a voice coil aperture with an internal cylindrical pole surface therein terminating in parallel annular front and back pole plate edges at said face surfaces; with a magnetically soft iron pole piece comprising a circular disc having flat parallel front and back face surfaces and an external cylindrical pole surface terminating in parallel annular edges at said face surfaces; said pole piece being concentric with the voice coil aperture and of thickness less than the pole plate; and positioned Wholly within the aperture with the front face surface of the pole piece spaced rearwardly from the front face of the pole plate and the rear face surface of the pole piece spaced forwardly from the rear face of said pole plate.

9. A magnetic circuit for a transducer comprising, in combination, a pole plate of magnetically soft iron having relatively at front and back face surfaces, and a voice coil aperture with an internal cylindrical pole surface therein terminating in parallel annular front and back pole plate edges at said surfaces; a magnetically soft iron pole piece comprising a circular disc having an external cylindrical pole surface terminating in parallel annular edges at said face surfaces, said pole piece being concentric with the Voice coil aperture and positioned in the Voice coil aperture; a return yoke of magnetically soft iron including a back plate parallel with and spaced from said pole plate, and a magnetic generator comprising a permanent magnet of aluminum-nickel-cobalt-iron alloy,

8 said magnet having parallel flat rear and forward pole faces, With the rear pole face abutting the back plate of the return yoke, and the forward pole face extending substantially to the rear face of the pole plate and abutting the baci"l face surface of the pole piece.

CYRUS E. HOEKSTRA.

JOHN C. RADCLIFFE.

REFERENCES CITED The following references are of .record in the le of this patent:

UNITED STATES PATENTS 

